1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include an endless fixing belt formed into a loop and a resistant heat generator provided inside the loop formed by the fixing belt to heat the fixing belt, to shorten a warm-up time or a time to first print (hereinafter also “first print time”). Specifically, the resistant heat generator faces the inner circumferential surface of the fixing belt across a slight gap. A pressing roller presses against a contact member also provided inside the loop formed by the fixing belt via the fixing belt to form a nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. As the recording medium bearing the toner image passes through the nip, the fixing belt heated by the resistant heat generator and the pressing roller apply heat and pressure to the recording medium to fix the toner image on the recording medium.
In the nip in the fixing device, since heavy pressure is exerted at a position between the fixing member and the pressing member, torque may be generated during a startup time and a recovery time from standby state. If the torque is strong, motors may be locked or gears may be broken.
To counteract this effect, it is possible to improve rotation and reduce friction resistance, a lubricant, such as grease, may be applied to an inner circumferential face of the endless fixing belt, at a portion facing a support member or the contact member.
However, viscosity of the lubricant is dependent on temperature, and thus the viscosity is significantly higher in a cooled state, due (for example) to the ambient temperature of the fixing device. Torque failure often occurs when the fixing device starts up in a state in which the ambient temperature is cool.
In order to prevent torque failure from occurring, the entire fixing device may be heated as the endless belt remains motionless to warm the lubricant on the endless belt. Then, rotation of the endless belt is restarted after the viscosity of the lubricant is sufficiently decreased by warming.
However, if the endless belt is heated in a non-rotation condition until the lubricant is warmed sufficiently, heating is time consuming and start-up time increases. More particularly, the start-up time of the endless belt under low-temperature conditions is significantly longer.
In addition, in a fixing device in which the heating member for the fixing member heats the fixing member not entirely and uniformly but only locally, it is difficult to transmit the heat to the lubricant covering the entire fixing device (particularly in the nip), and as result, the heating time until the fixing member start rotating is further increased.